Transcoding question

I have PLEX resident on my Windows 10 PC at home. I have several media files of all sorts, AVI, Matroska, MP4, etc. and so I just looked at 3 different devices streaming from my PLEX PC at home and all of them were transcoding both audio and video.
Is there one format/container that requires less transcoding than others?
In other words what file types can be played natively on more devices than any other? Clearly I want to keep transcoding to a minimum.

My Windows 10 PC is an older HP desktop with a 6 core AMD processor with an Operating speed: 2.9 GHz, Bus speed: 4000 MHz HT3. And it is connected to the internet via fiber optic at 50/50. It suits my needs but am afraid of maxing out the capabilities resulting in playback issues.

Im new to streaming PLEX and just trying to learn my limitations

Yes…
For 1080p
Encode
h264 / mp4 / AC3 5.1, AAC 2 channel

for 4K
Encode
h265 / mp4 / AC3 5.1, AAC 2 channel

That encoding will direct play in most cases.